1. Field of the Invention
The present invention relates to a separator for a valve regulated lead-acid battery which comprises a paper sheet manufactured by a wet papermaking process mainly comprising very fine glass fiber and has both functions as a retainer (holding material) for electrolytic solution and as a partition; and also relates to a valve regulated lead-acid battery using the same.
2. Description of the Related Art
With regard to a separator for valve regulated lead-acid battery, a separator comprising a paper sheet manufactured by a wet papermaking process constituted from very fine glass fiber only or from a sheet very fine glass fiber and a small amount of a binder only has been mostly used up to now. Recently however, there has been an increasing demand for mechanical strength as a result of automation of lines for assembling the batteries.
In the conventional separators which comprise a paper sheet manufactured by a wet papermaking process being constituted only from very fine glass fiber or only from very fine glass fiber together with a small amount of a binder, their mechanical strength is weak and it is difficult for them to comply with the automation in the assembly lines for batteries.
Further, in recent years, there have been many cases where a plate group (where a separator is incorporated between positive and negative plates) is assembled under high compression and inserted into a container in view of tendency of long life and high efficiency of a valve regulated lead-acid battery whereby, as a result of assembling of batteries under high compression, unevenness of the plate surface (or a big convex) and breakage of a separator by edges of plate grid (plate of an expanded grid) are becoming new problems. Therefore, it is necessary that a piercing strength (puncture strength) of a separator is high in order to prepare a separator where breakage of a separator due to unevenness of the plate surface or to edges of plate grid hardly happens even when assembling of batteries are conducted under high compression. It is difficult to prepare a separator having such a high piercing strength (puncture strength) in the case of the mainstream separator of a paper sheet prepared by a wet papermaking process being constituted only from very fine glass fiber or only from very fine glass fiber together with a small amount of a binder or, particularly, a separator being constituted only from very fine glass fiber.
Under such circumstances, the Patent Document 1 proposes a separator where very fine glass fiber is blended with a monofilament-form synthetic fiber such as polyethylene terephthalate fiber and a pulp-form organic fiber such as polyethylene synthetic pulp in a wet papermaking process whereby a compressive breakage strength and a piercing strength (puncture strength) are enhanced and breakage of a separator due to unevenness of plate surface or to edges of plate grid hardly happens even when assembling of batteries is conducted under high compression. However, even if a separator is constructed by such a material constitution, although the compressive breakage strength is enhanced, a tensile strength and a piercing strength (puncture strength) are not enhanced so much and, accordingly, breakage of a separator due to unevenness of plate surface or to edges of plate grid in the actual assembling of batteries under high compression is unable to be well prevented. Thus, in the separator of the Patent Document 1, although it is mentioned that a piercing strength (puncture strength) of a separator is able to be enhanced by addition of a pulp-form organic fiber such as synthetic pulp of polyethylene, short-circuit of separators and troubles in assembling are still generated depending upon the state of the plates of the assembling battery and the state of the battery assembly lines.
With regard to a binder which is able to give a tensile strength by bonding of glass fibers each other in a paper sheet manufactured by a wet papermaking process of very fine glass fiber, there have been already known an organic binder such as a resin emulsion or a pulp-form organic fiber mentioned in the Patent Document 1. However, in the separator using such a organic binder, the binder component forms a coat (film) as if it covers the surface of the glass fiber whereby there are problems that wetting of the glass fiber with respect to an electrolytic solution is inhibited, wetting of the separator with respect to an electrolytic solution is significantly inhibited and a property of the separator for holding the electrolytic solution lowers.    Patent Document 1: Japanese Patent Laid-Open No. 11/016,560 A